1. Field of the Invention
The present invention relates generally to an attachment for sampling an inert purge gas pressure at a weld site. More particularly, the invention relates to an attachment for use with an orbital welding apparatus for measuring the pressure of an inert purge gas flowing in a pipeline to be welded.
2. Background Description
Orbital welding machines have become quite popular for welding pipelines, and especially for the welding of stainless steel pipelines as are commonly run in large plants, for example, semiconductor manufacturing plants. Stainless steel pipelines are used to carry a great variety of gaseous and liquid materials through a plant, and the construction of such pipelines requires the welding of a large number of fittings and long lengths of high purity stainless steel pipe. The quality of weld required in such applications mandates a reproducible weld of uniformly high quality, and orbital welding machines have been developed for just such applications.
Orbital welding machines rely upon an essentially constant flow of an inert purge gas flowing through the pipeline at a low, but constant, pressure. However, in order to maintain a constant pressure of the gas, a very precise measuring technique is required. Typically, a low pressure gauge known as a Magnehelic gauge is used, providing a visual indication of pressures ranging from about zero to about ten inches of water. Most often, such gauges are connected to the pipeline being welded by means of a simple T-fitting temporarily inserted into the pipeline, with the stem of the fitting being connected directly to the gauge via a piece of tubing or hose. At one end the pipeline is connected to a source of inert gas, such as a gas cylinder, while the other end of the pipeline has a restrictor to reduce the size of the open end and provide a constant back pressure to the flowing inert gas.
Experience has shown that, while a good reading of the pressure may be obtained using a T-fitting, and a constant pressure can be maintained by closely watching the Magnehelic gauge, variations in the pressure in the pipeline nonetheless occur, causing problems with the quality of the weld. Moreover, the length of pipeline between the source of the gas and the restrictor varies from one weld to the next, and constant monitoring of the pressure is needed. Furthermore, as a weld progresses around the circumference of the pipeline or fitting, the gap between the two ends being welded is gradually closed, and the escape of gas through the butted ends of pipe gradually diminishes until the weld is completed. Since there is inherently some loss at the butted ends, there is a gradual increase in the gas pressure in the pipeline until the weld is completed. Under some conditions, if the pressure is not properly controlled as the weld progresses, the pressure can rise sufficiently to blow through the weld. The greater the distance between the weld site and the T-fitting, the more likely the weld is to blow through or the correct weld profile will not be maintained because of pressure difference at the weld site and the xe2x80x9cTxe2x80x9d location.
For example, the use of T-fittings also requires the installation and removal of the T-fitting at each weld site prior to each weld. However, a number of problems are also introduced by using such fittings for measuring the pressure of the inert gas. For example, the use of T-fittings also requires the removal and replacement of the T-fitting after a weld is completed and in preparation for the next weld. This can be not only time consuming, but allows for the introduction of impurities into the pipeline as the fitting is reused, or lays around ajob site until it is reused. The purity of the pipeline and the weld and welding process are of major importance in many plants, in particular in semi-conductor plants where very small amounts of trace elements can cause unacceptable variations in quality control. Thus, each time a T-fitting is reused, an additional possibility of contamination arises.
Additionally, since the T-fitting is installed in the pipeline, it may be installed at the joint to be welded prior to the welding or generally installed downstream of the weld site at the location of the next weld since it is not possible to monitor the pressure at the weld site during the welding, itself. However, the distance will usually vary between welds, such that the distance between the weld site and the T-fitting will not be constant from one weld to the next. As a result, more fluctuations in the purge gas pressure will occur, requiring still further compensation in the gas pressure. Also, the remote location may not have the same pressure as the weld site. In any event this measured pressure is used as a xe2x80x9cbestxe2x80x9d reference to maintain a proper pressure at the weld site during welding.
Also, there are times that the tubing and fittings are tacked together such as in spot welding of joints prior to the installing weld head or weld fixture block. In these cases there is no way to take a pressure reading at the weld site prior to welding. Also, all the joints are taped to seal them from contamination until ready to weld. Thus, there is no way that a T-fitting can then installed in the pipeline.
In the scenario in which the pipe and fittings have not been tacked, a measuring device capable of being placed around the pipe can be used inside an orbital weld head or weld fixture. The tungsten in the weld head, however, would interfere with the device if one would try to make the seal about the entire pipe.
A number of prior art pressure measuring devices have been developed in attempts to overcome these problems. However, these devices are subject to their own shortcomings. For example, existing devices create an annular chamber about the entirety of the joint to be welded.
It is therefore an object of the present invention to provide a pressure measuring device capable of taking pressure readings at the weld site before welding is performed.
It is another object of the present invention to provide a pressure measuring device that is simple to manipulate and use.
It is yet another object of the present invention to provide a pressure measuring device capable of providing a pressure measurement for a specific point along the perimeter of a joint to be welded.
The present invention is a device for measuring the pressure of a gas flowing in axially aligned abutting pipe members to be welded together at a joint. The device generally includes a collar having a groove formed in an inner surface thereof and a pressure sensing device in fluid communication with the groove, such as via a nipple and hose arrangement. The groove is formed to surround and seal only a portion of the perimeter of the joint to be welded, while the collar is formed to surround and seal an additional portion of the perimeter of the joint to be welded. The remaining portion of the perimeter of the joint to be welded may be sealed by tape or via seal means included in the pressure measuring device. It may also be sealed by extending the collar around the entire perimeter of the joint to be welded. Thus, the groove and the outer wall of the pipe members form a sealed cavity, the pressure inside of which provides an indication of the pressure of the flowing gas inside the pipe. In embodiments, the pressure measuring device includes a handle.